2013 Rear Camera/Headunit Questions

I have a 2013 Forester that I want to add a back up camera to. I have the older stock head unit/radio without any type of screen on it.
My questions are:
Do I need to upgrade the headunit or can I mount a flatscreen somewhere? If I need a new head unit what is the best for the lowest price?
Is there a plug and play rear view mirror that can be installed that has the 'rear view' when car is put in reverse?

I wanna get an idea of what I have to do as far as wiring and I don't wanna buy a new head unit if possible.
Just the simple DIY way to get the rear view in the drivers seat.
Thank you
"my first post"